1. Field of the Invention
The present general inventive concept relates to a confocal scanning microscope to measure critical dimensions of a semiconductor device and an image output device to perform a real time check in a production process of the semiconductor device and image output devices.
2. Description of the Related Art
Confocal microscopes are apparatuses for irradiating a sample with light having a certain wavelength, controlling the light to be reflected from the sample through a confocal aperture, such as a pin hole, and detecting the light only emitted from a focal plane of an objective lens using a photo-detector (PD), the principle of which is disclosed in Korean Patent Laid-Open No. 2002-0084786.
As disclosed in the above mentioned reference, since light reflected from a portion outside the focal plane of the objective lens of the confocal microscope does not pass through the pin hole, and hence, is not detected in the photo-detector, the confocal microscope has not only a high resolution in an optical axis direction but also a resolution higher than that of existing optical microscopes in an direction perpendicular to the optical axis direction. In addition, with the confocal microscope disclosed in the above reference, it is possible to observe a desired plane on the sample and obtain a three-dimensional image of the sample.
Owing to the high resolution and the capability of obtaining the three-dimensional image, confocal microscopes has been widely used in the fields of cell biology and semiconductor chip testing.
As one of methods for obtaining a two-dimensional plane image using such a confocal microscope, Japanese Patent Laid-Open No. Hei 06-018786 discloses a confocal microscope for scanning every point of a measurement area with light using point scanning of a television scan line system. The confocal microscope employs a method of deflecting light in two perpendicular axis directions using two optical deflectors, and scanning each point of the measurement area with the deflected light. However, such a confocal microscope has a problem in that it takes a long time to obtain the two-dimensional image due to a limited mechanical speed of the optical deflectors and a calculation load of serial signal processing.
As a method for obtaining higher image acquisition speed, compared to the method using the optical deflectors, U.S. Pat. No. 5,067,805 discloses a confocal scanning microscope using a Nipkow disk, the principle of which will be described with reference to FIG. 1.
Referring to FIG. 1, the confocal scanning microscope using the Nipkow disk includes a light source 1, a collimating lens 2 for transforming light emitted from the light source 1 into a parallel beam, a beam splitter 3 for changing a direction of the parallel beam incident from the collimating lens 2, a Nipkow disk 4 having a plurality of apertures formed therein such that only a portion of a beam incident from the beam splitter 3 passes therethrough, a motor 5 for rotating the Nipkow disk, a tube lens 6 for transforming a beam, which has passed through the Nipkow disk 4, into a parallel beam, an objective lens 7 for irradiating a sample 8 with the parallel beam incident from the tube lens 6, a first lens 9 for transforming the beam reflected from the sample 8 and passed through the Nipkow disk 4 into a parallel beam, and a second lens 10 for concentrating the beam passed through the first lens 9 onto a two-dimensional photo-detector 11 for acquisition of an image.
The light emitted from the source of light 1 becomes the parallel beam after passing through the collimating lens 2. This parallel beam illuminates a top surface of the Nipkow disk 4 after being reflected at the beam splitter 3. As shown in FIG. 2A, a plurality of apertures 4a each having a pin hole shape are distributed (formed) in the Nipkow disk 4. With this distribution of the apertures 4a, only a portion of the parallel beam irradiating the Nipkow disk 4 passes through the apertures 4a. A beam passed through the apertures 4a propagates at various angles by diffraction, thereby causing an effect as if a point of the light source is placed on each aperture 4a. The tube lens 6 and the objective lens 7 form an image on the sample 8 by irradiating the sample 8 with the beam passed through the apertures 4a. Only a plurality of point regions of an overall observation region on the sample 8, which correspond to the apertures 4a, are illuminated. In order to illuminate the overall observation region on the sample 8, positions of the apertures 4a must be varied. To this end, the motor 5 connected to a center of the Nipkow disk 4 rotates the Nipkow disk 4. When the positions of the apertures 4a are varied with respect to the sample 8 according to rotation of the Nipkow disk 4, the overall observation region on the sample 8 is illuminated.
The beam illuminating on the sample 8 is reflected from the sample 8 and passes through the objective lens 7 and the tube lens 6 for formation of an image on the Nipkow disk 4. At this time, some beam, reflected from a focal plane (f) of the objective lens 7, of the beam reflected from the sample 8, passes through the apertures 4a of the Nipkow disk 4, however, some portion of the beam, reflected from points deviated from the focal plane (f) in the optical axis direction, of the beam reflected from the sample 8, does not pass through the apertures 4a. This accounts for the so-called confocal principle through which high resolution in the optical axis direction can be obtained.
The beam passed through the apertures 4a is incident into the two-dimensional photo-detector 11 through the first lens 9 and the second lens 10 so that an image is formed on the photo-detector 11. As the positions of the apertures 4a are varied according to the rotation of the Nipkow disk 4 by the motor 5, a position on the photo-detector 11 at which the image is formed is varied. Accordingly, an optical signal is transported on the overall region of the two-dimensional photo-detector 11, so that a two-dimensional image can be at once obtained with respect to the sample 8.
FIG. 2B illustrates a shape of the Nipkow disk 4, where slit-shaped curve apertures 4b are formed. In the case of the confocal scanning microscope using the Nipkow disk 4 having the slit-shaped curve apertures 4b formed thereon, an illumination beam passing through the Nipkow disk 4 takes a line shape, and a region to be illuminated on the sample 8 also takes a line shape. When the Nipkow disk 4 is rotated, the illumination beam of the line shape for forming an image on the sample 8 is moved, and accordingly, the image of the line shape formed on the photo-detector 11 is also moved to obtain a two-dimensional image of the sample 8.
However, although the confocal scanning microscope using the Nipkow disk 4 has an advantage of an image acquisition speed higher than that of the confocal scanning microscope using the optical deflector, it has a problem of deterioration of the resolution in the optical axis direction since it illuminates not a point but a plurality of point regions or a line region on the sample for parallel processing of signals.
As shown in FIG. 3, the beam reflected from the focal plane (f) of the objective lens 7 is exactly concentrated on and exits through the apertures 4a of the Nipkow disk 4 through which the illumination beam has been emitted, after passing through the objective lens 7 and the tube lens 6. However, the beam reflected from the points deviated from the focal plane (f) and moved deeper in the optical axis direction is not exactly concentrated on the apertures 4a, forms an image before the apertures 4a, and then, passes through adjacent apertures 4a as well as the apertures 4a through which the illumination beam is emitted. The beam illuminated on the adjacent apertures 41 acts as a kind of noise, which deteriorates optical performance in the optical axis direction.
FIG. 4 is a graph showing a resolution in an optical axis direction when a size of the aperture in confocal scanning microscopes using a single aperture and multiple apertures is changed. In order to obtain the amount of beam of a range within which an image can be measured, it is required to increase the size of the aperture over a prescribed size. However, it can be seen from FIG. 4 that a resolution value in the confocal scanning microscope using the multiple apertures is abruptly increased as the size of the aperture is increased, which results in deterioration of performance of the confocal scanning microscope.